Large Format Lenses
The lens is what allows light to enter your camera and hit the film. Technically, a lens is not necessary and a pinhole would work to do this same job, but with a lens you have much more control over the image produced.
A lens allows more light to enter over a larger area than a pinhole, which means the exposure time can be significantly reduced, which is far more practical.
A lens also allows you to focus the image; with a pinhole there is no defined focal length and cannot focus at all. A lens gives you so much more control in that sense, allowing you to compose the image with many more options.
The physics behind focusing
When the lights rays pass through one medium into a different medium, in this example air and glass, the speed of the light waves change at the point at the second medium. So when light enters through glass, the speed the light travels at changes when it hits the glass. If the glass is at a certain angle, the light will follow that and change direction. This is refraction. However, with the use of a lens the amount of refraction can be controlled through changing the shape and build of the glass within the lens to help the user focus the image.
Focal Lengths
The focal length of a lens measures the distance between the light rays converge in the centre of the lens to the sensor, or in this case of large format, the film within the camera.
"The focal length of a lens determines its angle of view, and thus also how much the subject will be magnified for a given photographic position." - Cambridge In Colour
This means that depending on the focal length the subject will appear a different size.
Wide angle lenses have short focal lengths, whereas longer lenses such as telephoto lenses have a much longer focal length.
While I couldn't find a visual of this for large format cameras and lenses, the principle is very similar. The focal length is simply the distance between the point of where all the light rays converge and the piece of film, rather than the sensor.
Image Circle
The image circle of a lens refers to the size of the whole circle that the lens creates. When using a lens, you usually use one designed for the size of the film or sensor that you're using, and so the whole circle covers the entirety of the film/sensor. Large format lenses will work with any camera, given that the lens has an appropriate adaptor for the camera in use. This is because large format cameras have the most amount of area to cover because the film is so large, and so the image circle is much bigger. When used on a camera that uses smaller film, the lens would easily let in enough light to cover the area of the smaller film, only it wouldn't be able to show as much of the scene as the large format camera would due to the smaller surface area of the film. This is called cropping factor. However, if you used a 35mm film lens on a large format camera, the amount of light let in by the lens wouldn't be adequate enough for the whole of the large format film to be exposed to the light, and therefore vignetting or dark rounded edges on the corners of the image (the lens edge) would occur.
As large format isn't just one size, finding an appropriate lens can be harder than finding one for a 35mm camera as the size of the film is always the same. For example, you may find a suitable lens for 4x5 film, but using it on 8x10 film which is double the size, may show vignetting or the circle edges as the area of light isn't big enough for the larger film.
Angle Of Coverage
This measures the image circle of the lens, but is not to be confused with the angle of view. The angle of coverage is about the image-forming cone within the camera lens.
What we can see here is how much of an image will be on the film depending on film size. The image circle is the image that is taken into the lens as a cone. When the film is bigger than the image circle, like the 4x5 in this example, it would make the angle of coverage too small for the film.
The angle of coverage will vary depending on the lens and how much information it can take in through the lens.
A lens allows more light to enter over a larger area than a pinhole, which means the exposure time can be significantly reduced, which is far more practical.
A lens also allows you to focus the image; with a pinhole there is no defined focal length and cannot focus at all. A lens gives you so much more control in that sense, allowing you to compose the image with many more options.
The physics behind focusing
When the lights rays pass through one medium into a different medium, in this example air and glass, the speed of the light waves change at the point at the second medium. So when light enters through glass, the speed the light travels at changes when it hits the glass. If the glass is at a certain angle, the light will follow that and change direction. This is refraction. However, with the use of a lens the amount of refraction can be controlled through changing the shape and build of the glass within the lens to help the user focus the image.
Focal Lengths
The focal length of a lens measures the distance between the light rays converge in the centre of the lens to the sensor, or in this case of large format, the film within the camera.
"The focal length of a lens determines its angle of view, and thus also how much the subject will be magnified for a given photographic position." - Cambridge In Colour
This means that depending on the focal length the subject will appear a different size.
Wide angle lenses have short focal lengths, whereas longer lenses such as telephoto lenses have a much longer focal length.
While I couldn't find a visual of this for large format cameras and lenses, the principle is very similar. The focal length is simply the distance between the point of where all the light rays converge and the piece of film, rather than the sensor.
Image Circle
The image circle of a lens refers to the size of the whole circle that the lens creates. When using a lens, you usually use one designed for the size of the film or sensor that you're using, and so the whole circle covers the entirety of the film/sensor. Large format lenses will work with any camera, given that the lens has an appropriate adaptor for the camera in use. This is because large format cameras have the most amount of area to cover because the film is so large, and so the image circle is much bigger. When used on a camera that uses smaller film, the lens would easily let in enough light to cover the area of the smaller film, only it wouldn't be able to show as much of the scene as the large format camera would due to the smaller surface area of the film. This is called cropping factor. However, if you used a 35mm film lens on a large format camera, the amount of light let in by the lens wouldn't be adequate enough for the whole of the large format film to be exposed to the light, and therefore vignetting or dark rounded edges on the corners of the image (the lens edge) would occur.
As large format isn't just one size, finding an appropriate lens can be harder than finding one for a 35mm camera as the size of the film is always the same. For example, you may find a suitable lens for 4x5 film, but using it on 8x10 film which is double the size, may show vignetting or the circle edges as the area of light isn't big enough for the larger film.
Angle Of Coverage
This measures the image circle of the lens, but is not to be confused with the angle of view. The angle of coverage is about the image-forming cone within the camera lens.
The angle of coverage will vary depending on the lens and how much information it can take in through the lens.
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